USING ICHNOFOSSILS AND ICHNOCOENOSES TO INTERPRET PALEOENVIRONMENTAL AND PALEOECOLOGICAL SETTINGS OF THE UPPER JURASSIC MORRISON FORMATION, WESTERN INTERIOR BASIN, USA

Ichnofossils of at least seventy-five types documented in the Upper Jurassic Morrison Formation demonstrate that highly diverse and abundant plants, invertebrates, and vertebrates occur throughout most of the strata. Invertebrate ichnofossils preserve the most environmentally and climatically sensitive organism behavior. Ichnofossils in terrestrial deposits record biotic processes involved in soil formation, including those that burrow to the water table. Traces record soil moisture and water-table levels of those deposits. Ichnofossils in freshwater deposits preserve evidence of environmental stability, salinity, and seasonality of water bodies. Ichnofossils categorized as epiterraphilic, terraphilic, hygrophilic, and hydrophilic indicate the moisture regime of the substrate where they were constructed. The ichnofossils are zoned vertically with respect to physical, chemical, and biological factors in the environment as well as ecological relationships that controlled their distribution and abundance. Thus, unique suites of ichnocoenoses occur in alluvial, lacustrine, palustrine, and eolian environments recorded by strata of the Morrison Formation.

Morrison ichnofossils indicate environmental, ecological, and climatic settings across the basin through time. Marginal-marine, tidal to brackish-water ichnofossils are restricted mainly to the Windy Hill Member. Deeper and larger insect nests indicate more seasonal distribution of precipitation and rainfall. Shallower and smaller insect nests indicate either dry or wet substrate conditions depending on the nest architecture and paleopedogenic and sedimentologic character of the substrate. Communities of perennial, freshwater bivalve traces are abundant in the Tidwell and Brushy Basin Members but to a lesser extent in the Salt Wash Member. Shallow crayfish burrows restricted to channel bank and proximal alluvial deposits in the Salt Wash, Recapture, and Brushy Basin Members, indicate a water-table level close to the surface (< 1 m). Sauropod, theropod, pterosaur, and other vertebrate tracks and trackways occur throughout the Morrison Formation in alluvial, lacustrine, and transitional-marine shoreline deposits.